Methyl parathion is used in the agricultural industry as a contact and stomach poison with broad-spectrum insecticidal activity and some efficacy against mites. It is sold as a wettable power or emulsifiable concentrate for foliage application. Several formulations contain combinations of methyl parathion and ethyl parathion as well as other registered pesticides. There are 62 crops on which methyl parathion is registered for use, but over 90% of the total volume used in 1974 was on cotton. It is used to some extent in California for mosquito control.
A bioassay of methyl parathion for possible carcinogenicity was conducted by administering the test chemical in feed to F344 rats and B6C3F1 mice.
Groups of 50 rats of each sex were administered methyl parathion at one of two doses, initially either 62.5 or 125 ppm. These doses were maintained for 102 weeks for the females; however, due to decreased mean body weight gain in the dosed males, the low and high doses for the males were reduced after 37 weeks to 20 and 50 ppm, respectively, and administration at the lowered doses was continued for 65 weeks. The time-weighted average doses for the male mice were 35 and 77 ppm, respectively, for the low- and high-dose groups. Matched controls consisted of 20 untreated mice of each sex. All surviving mice were killed at the end of administration of the test chemical.
Mean body weights of the dosed male and female rats and mice were lower than those of the corresponding controls throughout the bioassay and were dose related. Survival was unaffected in both species except for an increase in mortality in the high-dose female rats, in which 46% of the animals were alive at the end of the study.
No tumors occurred in any of the groups of rats or mice of either sex at incidences that were significantly higher in the dosed groups than in the corresponding control groups.
It is concluded that under the conditions of this bioassay, methyl parathion was not carcinogenic for F344 rats or B6C3F1 mice of either sex.